From the viewpoint of reducing the weight of automobile bodies and of automobile crash safety, high-strength steel sheets are being increasingly applied to various structural members and reinforcing members of automobiles. For practical use of the high-strength steel sheets, these high-strength steel sheets have a need for improved press-formability. Particularly, to form a high-strength steel sheet into a component having a complex shape, it is necessary that not only one of the properties of the steel sheet such as ductility and hole expandability be good but also both the properties be good.
On the other hand, a high strength steel sheet that is reduced in thickness sees significant impairment in shape fixability. For this reason, it is a widespread practice to perform press forming by predicting change in shape of pressed parts separated from the mold so as to design the press mold in expectation of the change in shape. Here, if the tensile strength of a steel sheet is changed significantly, the actual change in shape deviate from the expected change based on the assumption that the tensile strength would remained unchanged, which leads to shape defects, making indispensable the procedure of subjecting the pressed parts one by one to sheet processing for shape correction, with the result that mass-production efficiency is significantly deteriorated. In view of this, there has been a demand for high strength steel sheets with minimized difference in strength, that is, having excellent material homogeneity.
Particularly, in a thin high-strength steel sheet having a tensile strength (TS) of more than 1,450 MPa, residual stress after press forming and hydrogen entering from the environment may cause delayed fracture. Therefore, when a high-strength cold-rolled steel sheet is used as the above-described thin steel sheet for automobiles, it is necessary that the steel sheet have high press-formability, i.e., be excellent in ductility and hole expandability (hereinafter may be referred to also as stretch flangeability), and also have excellent material homogeneity and excellent delayed fracture resistance.
Various techniques for achieving formability and delayed fracture resistance simultaneously are previously known. For example, Patent Literature 1 discloses a high-strength cold-rolled steel sheet having excellent bendability and delayed fracture resistance. This steel sheet has a prescribed chemical composition comprising Si: 1.0 to 2.0% and has a metal structure in which the volume fraction of a tempered martensite phase is 97% or more and the volume fraction of a retained austenite phase is less than 3% (in all regions except for a region within a depth of 10 μm from the surface of the steel sheet). This steel sheet has a tensile strength of 1,470 MPa or more, and the ratio of its 0.2% proof stress to the tensile strength is 0.80 or more. In Patent Literature 1, it is stated that the addition of Si allows the work hardening ability of the tempered martensite phase to be improved and fine carbides to be dispersed uniformly in the structure, so that a cold-rolled steel sheet having a very high tensile strength of 1,470 MPa or more and also having high bendability and excellent delayed fracture resistance can be obtained.
Patent Literature 2 discloses a high-strength cold-rolled steel sheet having excellent hydrogen embrittlement resistance and formability. This steel sheet has a prescribed chemical composition comprising V: 0.001 to 1.00% and has a structure in which the area fraction of tempered martensite is 50% or more (including 100%) and the remainder is ferrite. The distribution state of precipitates in the tempered martensite is as follows. The number of precipitate particles having an equivalent circular diameter of 1 to 10 nm per 1 μm2 in the tempered martensite is 20 or more, and the number of V-containing precipitate particles having an equivalent circular diameter of 20 nm or more per 1 μm2 in the tempered martensite is 10 or less. In Patent Literature 2, it is stated that, by appropriately controlling the area fraction of the tempered martensite and the distribution state of the V-containing precipitate precipitated in the tempered martensite in a tempered martensite single-phase structure or a two-phase structure including the ferrite and the tempered martensite, stretch flangeability is improved while hydrogen embrittlement resistance is ensured.